PROJECT SUMMARY Older adults exhibit a substantial reduction in propulsive power generation. This change in walking mechanics is associated with slower walking speeds and greater metabolic energy expenditure ? features that negatively affect independence and quality of life. Age-related reductions in propulsive power are often attributed to reduced ankle mechanical power during the ?push-off? phase of walking. Yet, strengthening the calf muscles has been an ineffective strategy for improving push-off intensity, walking speed, and/or metabolic cost, motivating the need for new and modifiable targets for preserving mobility in our aging population. Recent studies have determined that the foot, independent of the ankle, plays a critical role in governing push-off intensity. The interaction between active structures (e.g. muscles within the foot) and passive structures (e.g. plantar aponeurosis ? an elastic structure spanning the bottom of the foot) of the foot are vital to economic locomotion, but it is unclear how this interaction facilitates forward propulsion, or how this interaction is changed with age. We propose that age-related changes in passive and active contributions to foot stiffness may contribute to reductions in push-off intensity in older adults, both directly via deficits in foot power and indirectly via the misappropriation of ankle power. Aim 1: Our proposal will leverage a novel dual-probe ultrasound imaging technique to determine the role of foot stiffness and plantar intrinsic muscle contractile dynamics in governing foot-ankle interaction dynamics in young adults. Aim 2: We will then characterize the local energetic and mechanical response to changes in plantar aponeurosis stiffness by closely integrating our experimental data from Aim 1 into a computational model of the lower extremity. Aim 3: Finally, we will determine the effects of age on foot stiffness and its role in walking economy and functional mobility. This study has the potential to influence a paradigm shift in our biomechanical understanding and clinical management of age-related mobility impairment. Moreover, our findings will have immediate impact on targeted mobility intervention opportunities and innovation in the design of wearable integrated foot and ankle devices for enhanced mobility toward improving the health and welfare of our aging population. Finally, our technological advancements in musculoskeletal imaging will revolutionize the use of in vivo ultrasound during functional locomotor behavior, with broad implications in humans and other animals.